I've had both the SuperZero 2.0 and P4 in my house... I can tell you that the SZ2.0 does not get anywhere near 90Hz with any authority, and really needs a 120Hz or higher xover.
The P4 has a bit better low end, but really needs rear wall or desktop reinforcement to bring the bass up to match the upper part of the spectrum... which you mentioned you'd be doing.
I've also had the S30 in my home, and they are MUCH more capable - both in bass extension and SPL - than the other 2 options. Also, despite the specs saying it's a 4.5" woofer, it is noticeably larger than the woofer in the P4 and SZ2.0. It's close in size to what mose companies call a 5.25" woofer (yes, I've measured).
Cambridge does recommend at least 3-4" clearance behind them, although I think they'd benefit from a little more than that. These speakers don't have alot of baffle step compensation in the xover, so the tweeter is shelved up just a bit relative to the woofer, so having them close to the wall is not necessarily bad... as long as they aren't jammed right up against it.
The Mordaunt Carnival 2 looks to be very similar to the S30, so the front porting may be helpful if you need them really close to the rear wall.
If the S30 is a viable option - check AVHiFi,com. They have a couple open box pairs at a nice savings... and they have an active 10% off coupon floating around the net that will drop the price a bit more...